Heretofore, olefin (co)polymers have widely been used as general-purpose resins in many fields, but among these olefin (co)polymers, ethylene (co)polymers have the following problems and thus their improvement has been strongly desired. For example, in a linear low-density polyethylene (L-LDPE) and a high-density polyethylene (HDPE), the activation energy of fluidization in a melting state is small, so that they are poorer in moldability as compared with a low-density polyethylene (LDPE), and in particular, a high-molecular weight compound has a substantial problem that its moldability is poor. Furthermore, the ethylene (co)polymer has the problem of neck-in at the time of the molding of sheets and films, and moreover, the linear low-density polyethylene has a problem that transparency and heat-sealing properties are poor.
As a method for solving such a problem, for example, there has been disclosed an olefin copolymer having long-chain branches obtained by using an .alpha.,.omega.-diene compound and a cyclic endomethylene diene compound (Japanese Patent Application Laid-open No. 34981/1972). However, in this olefin copolymer, a diene component is concerned with the long-chain branches and a crosslinking reaction simultaneously occurs, and a gel is generated at the time of film molding. In addition, melt properties inversely deteriorate, and a control range is extremely limited. Moreover, a copolymerization reactivity is also low, and owing to the formation of low-molecular weight compounds, physical properties inconveniently deteriorate.
Furthermore, another preparation method has been disclosed which is characterized in that when a non-conjugated diene compound is copolymerized with an olefin, the polymerization is carried out in two steps, and the content of a non-conjugated diene compound unit in a high-molecular weight portion is higher than that of the non-conjugated diene compound unit in a low-molecular weight portion (Japanese Patent Application Laid-open No. 56412/1984). However, in this method, a long-chain branch is introduced into a high-molecular weight component, and therefore the increase in the molecular weight due to crosslinking is noticeable. In consequence, insolubilization, nonfusion and gelation simultaneously occur with a high probability, and a control range is narrow and a copolymerization reactivity is also low, so that physical properties inconveniently deteriorate owing to the formation of the low-molecular weight compound.
In addition, an ethylene-.alpha.-olefin-1,5-hexadiene copolymer obtained by the use of a metallocene-aluminoxane catalyst has been disclosed (Japanese Patent Application Disclosure No. 501555/1989). However, in this copolymer, a molecular-weight distribution is narrow, which is disadvantageous to blow molding and film formation, and the copolymer is inconveniently devoid of thermal stability.
Moreover, also with regard to a propylene polymer, a resin design has been made to sufficiently utilize characteristics peculiar to the propylene polymer by imparting novel characteristics (particularly melt tension), with the intention of achieving the wider application of the propylene polymer. For example, in Japanese Patent Application Laid-open Nos. 185490/1993, 194659/1993, 194778/1993, 194793/1993, 200849/1993, 202137/1993, 202143/1993, 202219/1993, 202237/1993, 202238/1993, 202248/1993, 209062/1993, 212771/1993, 212774/1993, 214178/1993, 220829/1993, 222121/1993, 222122/1993, 222251/1993, 228995/1993, 237930/1993 and 239232/1993, there have been disclosed techniques of imparting the sufficiently improved melt tension to a propylene polymer and a resin composition containing this polymer by combining a preliminary polymerization catalyst with a preliminary polymerization method. However, in the techniques disclosed in these publications, the preliminary polymerization operation which is carried out prior to a main polymerization comprises at least 3 steps, and so the operation is intricate. Besides, its reactivity is usually poor, and a chain non-conjugated diene which might bring about cyclization and a crosslinking reaction during the reaction is inconveniently used.